1. Field of the Invention
The present invention relates generally to the field of medical devices for implantation into humans. More particularly, it concerns methods for processing biological tissues for use as bioprosthetic devices.
2. Description of the Related Art
Bioprostheses are devices derived from processed biological tissues to be used for implantation into humans. The development of such devices originated as an attempt to circumvent some of the clinical complications associated with the early development of the mechanical heart valve, and has since resulted in a rapid proliferation of bioprosthetic devices for a variety of applications. Examples of some of the bioprostheses currently used or under development include heart valves, vascular grafts, biohybrid vascular grafts, ligament substitutes pericardial patches, and others.
The primary component of the biological tissues used to fabricate bioprostheses is collagen, a generic term for a family of related extracellular proteins. Collagen molecules consists of three chains of poly(amino acids) arranged in a trihelical configuration ending in non-helical carboxyl and amino termini. These collagen molecules assemble to form microfibrils, which in turn assemble into fibrils, resulting in collagen fibers. The amino acids which make up the collagen molecules contain side groups, including amine (NH.sub.2), acid (COOH) and hydroxyl (OH) groups, in addition to the amide bonds of the polymer backbone, all of which are sites for potential chemical reaction on these molecules.
Because collagenous tissues degrade very rapidly upon implantation into a host recipient, it is necessary to stabilize the tissue if it is to be used clinically. Chemical stabilization by tissue cross-linking, also referred to as tissue fixation, has been achieved using bifunctional and polyfunctional molecules having reactive groups capable of forming irreversible and stable intramolecular and intermolecular chemical bonds with the reactive amino acid side groups present on the collagen molecules.
Molecules having two or more reactive aldehyde groups, also referred to herein as polyfunctional aldehydes, represent the most commonly used class of agents for cross-linking biological tissues. The most widely used of these polyfunctional aldehydes is the five carbon molecule, glutaraldehyde, which has an aldehyde at each end of a linear aliphatic chain. The aldehyde groups of glutaraldehyde and other like molecules can react under physiological conditions with the primary amine groups of collagen molecules to produce the desired cross-linked tissue.
Despite its widespread use, there are a number of drawbacks associated with tissue cross-linking with polyfunctional aldehydes. For example, under typical storage conditions, these compounds are generally self-reactive and will rapidly reach an equilibrium in which numerous polymeric and other species are present (see, for example, Khor (1997) and references cited therein). As a result, a pure solution of a monomeric polyfunctional aldehyde will become highly heterogeneous over time. Indeed, it is these heterogeneous solutions that have been conventionally used in the art for cross-linking biological tissues. Unfortunately, the properties of tissues cross-linked with these solutions may suffer as a result of this heterogeneity, as further described below.
An important issue when using polyfunctional aldehydes for treating biological tissues relates to the toxicity of the resulting cross-linked material. This toxicity is not completely understood, but may result from more than one mechanism. For example, the polymeric products of glutaraldehyde that are present in a heterogeneous glutaraldehyde solution can depolymize in vivo, causing the release of toxic monomeric glutaraldehyde into the recipient of the bioprosthesis. Such leaching of glutaraldehyde can prevent the cellular growth on the bioprosthesis following implantation that is necessary for long term biocompatibility. In addition, because of the presence of polymeric species of glutaraldehyde, there is an undesirable abundance of free aldehyde groups present within the cross-linked tissue. These free, unreacted aldehyde groups are also believed to contribute to the toxicity of aldehyde cross-linked tissues, for example, by reacting with cellular proteins present on the endothelial cells that must proliferate on and around the tissue after implantation.
Another significant drawback associated with polyfunctional aldehyde cross-linking is the propensity of the treated tissues to undergo calcification. For instance, calcification appears to represent the predominant cause of failure of glutaraldehyde-fixed devices (Golomb et al., 1987; Levy et al., 1986; Thubrikar et al., 1983; Girardot et al., 1995). It is believed that the presence of polymeric forms of glutaraldehyde in the cross-linked tissue may contribute to such calcification, possibly by serving as a physical point of calcification (Thoma et al., 1987).
Thus, it is a significant disadvantage that free aldehyde groups are present in tissue that has been cross-linked with heterogeneous polyfunctional aldehyde solutions. The present invention is directed to overcoming or at least reducing the effects of one or more of the problems set forth above. In particular, a method has been developed greatly reduces the toxicity and increases the biocompatibility of tissues that have been cross-linked with polyfunctional aldehydes.